Tungsten isotopic evidence for disproportional late accretion to the Earth and Moon

• Published in: Nature (London) Vol. 520; no. 7548; pp. 530 - 533
• Main Authors: Touboul, Mathieu, Puchtel, Igor S., Walker, Richard J.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 23-04-2015; Nature Publishing Group
• Subjects: 704/445/209; Accretion; Atomic properties; Chemical properties; Comparative analysis; Composition; Cosmic rays; Crystallization; Discovery and exploration; Earth; Earth mantle; Hafnium; Humanities and Social Sciences; Identification and classification; Isotope analysis; Isotopes; letter; Lunar petrology; Magma; Mantle; Measurement; Metals; Meteors & meteorites; Methods; Moon; multidisciplinary; Ratios; Science; Sciences of the Universe; Systematics; Tungsten
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature14355

Examination of three lunar samples reveals that the Moon’s mantle has an excess of the tungsten isotope
182
W of about 20 parts per million relative to the present-day Earth’s mantle; this suggests that the two bodies had identical compositions immediately following the formation of the Moon, and that the compositions then diverged as a result of disproportional late accretion of chondritic material to the Earth and Moon.
Late accretion to Earth and Moon
Two papers published in this issue of
Nature
present precise measurements of tungsten isotope composition in lunar rocks that are best explained by the Earth and Moon having had similar composition immediately following formation of the Moon, and then having diverged as a result of disproportional late accretion of material to the two bodies. Mathieu Touboul
et al
. found small
182
W excess of about 21 parts per million relative to the present-day Earth's mantle in metals extracted from two KREEP-rich Apollo 16 impact-melt rocks, while Thomas Kruijer
et al
. measured tungsten isotopes in seven KREEP-rich whole rock samples that span a wide range of cosmic ray exposure ages, and found a
182
W excess of about 27 parts per million over the present-day Earth's mantle.
Characterization of the hafnium–tungsten systematics (
182
Hf decaying to
182
W and emitting two electrons with a half-life of 8.9 million years) of the lunar mantle will enable better constraints on the timescale and processes involved in the currently accepted giant-impact theory for the formation and evolution of the Moon, and for testing the late-accretion hypothesis. Uniform, terrestrial-mantle-like W isotopic compositions have been reported
1
,
2
among crystallization products of the lunar magma ocean. These observations were interpreted to reflect formation of the Moon and crystallization of the lunar magma ocean after
182
Hf was no longer extant—that is, more than about 60 million years after the Solar System formed. Here we present W isotope data for three lunar samples that are more precise by a factor of ≥4 than those previously reported
1
,
2
. The new data reveal that the lunar mantle has a well-resolved
182
W excess of 20.6 ± 5.1 parts per million (±2 standard deviations), relative to the modern terrestrial mantle. The offset between the mantles of the Moon and the modern Earth is best explained by assuming that the W isotopic compositions of the two bodies were identical immediately following formation of the Moon, and that they then diverged as a result of disproportional late accretion to the Earth and Moon
3
,
4
. One implication of this model is that metal from the core of the Moon-forming impactor must have efficiently stripped the Earth’s mantle of highly siderophile elements on its way to merge with the terrestrial core, requiring a substantial, but still poorly defined, level of metal–silicate equilibration.